A gas turbine is a power plant, which produces an enormous amount of power for its size and weight. Due to their efficiency in power production, gas turbines have found increasing service in the past 40 years in the power industry, both among utilities and merchant plants, as well as in oil exploration and production, oil refining, and petrochemical industries.
In the utility industry, the past decade has seen unprecedented consumption of electricity that in many areas of the world frequently exceeds electricity supplies, leading to the possibility of power outages. In certain areas of the world, such electricity deficits have risen to crisis levels. Consequently, there is increasing pressure on utilities and independent power producers to become more efficient in producing power to increase their capacity to meet the growing demand for electricity.
In the case where gas turbine engines are used to produce power, modernizing existing fleets of gas turbines is emerging as an economically attractive solution. To that end, prior modernization efforts include, but are not limited to, improving metallurgical characteristics of gas turbine engine materials, improving combustion characteristics during operation of gas turbines, improving cooling characteristics within gas turbine engines, and improving airflow characteristics during operation of gas turbine engines.
In the area of improving airflow characteristics, parasitic loss of valuable high-temperature high-pressure combustion gases expanding through the turbine section of gas turbine engines is of paramount concern. Such parasitic loss is especially pronounced through clearance gaps located between turbine rotor blades and the outer casing of the turbine section, resulting in substantial reduction in gas turbine efficiency. Consequently, several modernization efforts have focused on reducing such parasitic loss through these clearance gaps.
For example, turbine rotor blades often have shrouds that form a band around the perimeter of a row of turbine rotor blades attached to a rotating disk (turbine wheel). These shrouded turbine rotor blades effectively reduce gas leakage around the tips of the blades and reduce blade vibration. Consequently, the use of shrouds on turbine rotor blades increases the efficiency of the gas turbine unit by improving airflow characteristics. However, in time, centrifugal forces, high temperatures and gas pressure differentials across the top and bottom of the shroud tend to “curl” and deflect the shrouds, resulting in excessive blade deformations (e.g. “creep”; i.e. elongation of the blade), increased parasitic loss, and may ultimately lead to catastrophic failure of the entire gas turbine unit.
To reduce the significance of blade deflection, it is common practice to scallop the turbine rotor blade shrouds, i.e., remove unsupported portions of the shroud. However, scalloping increases the parasitic loss of the combustion gases around the turbine rotor blades. Another common practice is to incorporate tip shroud rails in the turbine rotor blade shrouds that stiffen the shroud and form a labyrinth with matching rails of stationary shrouds attached to an outer casing of the turbine section. However, tip shroud rails currently incorporated on the turbine blade shroud have a trapezoidal profile, tapering in width as measured from the base of the rail to the top of the rail. Such a profile allows for ease of casting, but is the least effective in stiffening the shroud and in retarding parasitic loss of expanding combustion gases through the clearance gaps.
Another effort to reduce parasitic loss is the use of a honeycomb rub strip mounted to the stationary shroud, which, in turn, is supported by an outer casing. Honeycomb rub strips operate as labyrinth seals, which reduces the amount of parasitic loss of the expanding combustion gas, thereby increasing the efficiency of the gas turbine. However, the use of honeycomb rub strips requires the use of hardened cutter teeth attached to the tip shroud rails to cut a path through the honeycomb rub strip. These cutter teeth often damage the honeycomb rub strip and significant portions of the tip shroud rail grind down during operation of the gas turbine, resulting in partial or complete loss of the rails. Consequently, parasitic loss of expanding combustion gases between clearance gaps dramatically increases and the turbine rotor blades suffer accelerated deflection, resulting in substantial power loss and ultimately catastrophic failure of the gas turbine unit.
Although these efforts are advances in the art, there is still a need to increase gas turbine efficiency without compromising the long term mechanical reliability of the gas turbine. It has been found that generating a vortex or air dam at the leading edge of the tip shroud rails advantageously reduces parasitic air loss by restricting the flow of the combustion gases through the clearance gaps and redirecting the flow of the combustion gases to the airfoils of the turbine rotor blades, eliminating the need to use honeycomb rub strips to reduce parasitic air loss.
It has also been found that employing tip shroud rails that taper in width as measured from the top of the tip shroud rail to the base of the tip shroud rail dramatically reduces parasitic loss of expanding combustion gases between clearance gaps over existing tip shroud rails that taper in width as measured from the base of the tip shroud rail to the top of the tip shroud rail.
It has also been found that employing tip shroud rails that have a concave upstream surface dramatically reduces parasitic loss of expanding combustion gases between clearance gaps over conventional tip shroud rails.
It has also been found that employing tip shroud rails that have a convex upstream surface dramatically reduces parasitic loss of expanding combustion gases between clearance gaps over conventional tip shroud rails.